Expectations for a fuel cell as a power supply for portable electronic devices supporting the information-oriented society have recently been growing from a point of view of high power generation efficiency and high energy density as a stand-alone power generation apparatus. The fuel cell generates electric power through such reaction as electrochemical oxidization of a reducing agent (such as hydrogen, methanol, ethanol, hydrazine, formalin, and formic acid) at an anode electrode and electrochemical reduction of an oxidizing agent (such as oxygen in air) at a cathode electrode.
The fuel cell, however, suffers a problem of low power density per volume. In seeking for a fuel cell having a smaller size and lighter weight, a fuel cell achieving high power density has been desired.
In general, except for a high-temperature fuel cell such as a molten carbonate cell, such conventional fuel cells as a polymer electrolyte fuel cell, a solid oxide fuel cell, a direct methanol fuel cell, and an alkaline fuel cell are based on a unit cell as a constituent unit, that has a planar structure obtained by stacking an anode separator in which an anode flow channel for supplying a reducing agent is formed, an anode current collector for collecting electrons from an anode catalyst layer, an anode gas diffusion layer, an anode catalyst layer, an electrolyte membrane, a cathode catalyst layer, a cathode gas diffusion layer, a cathode current collector for feeding electrons to the cathode catalyst layer, and a cathode separator in which a cathode flow channel for supplying an oxidizing agent is formed in this order. Each unit cell produces a high current with a low voltage.
In addition, it is also general to provide the anode separator and the cathode separator not only with a role to supply the reducing agent and the oxidizing agent separately to the anode catalyst layer and the cathode catalyst layer respectively but also with a role as the anode current collector and the cathode current collector respectively, by employing an electrically conductive material as a material for these separators.
Normally, as individual unit cells are low in voltage, a fuel cell is formed as a fuel cell stack in which a plurality of unit cells are stacked to be able to output a high voltage. Here, the plurality of unit cells are stacked such that the anode electrode of a unit cell is electrically in contact with the cathode electrode of a unit cell adjacent thereto.
In such a layered fuel cell stack, intimate electrical contact between the layers should be maintained. If contact resistance becomes high, internal resistance of the fuel cell becomes high and overall power generation efficiency is lowered. In addition, the fuel cell stack usually includes a sealing material for sealing each of the reducing agent and the oxidizing agent in each separator, and each layer should be pressed with quite strong force in order to ensure sealing performance and electrical conductivity. Accordingly, fastening members for pressing each layer such as a presser, a bolt and a nut have been required, which resulted in a large and heavy fuel cell stack and low power density.
Further, as each separator requires a flow channel for uniformly supplying the reducing agent or the oxidizing agent to each unit cell all over a plane of the catalyst layer, the separator is large in thickness, which caused lower power density. If the flow channel in the separator is narrowed to make the thickness of the separator smaller, pressure loss becomes greater. Then, it is inevitable to use large auxiliary equipment such as a pump or a fan for supplying the reducing agent and the oxidizing agent, and in addition, power consumption by the auxiliary equipment also increases. Consequently, power density of a fuel cell system as a whole is lowered.
In order to solve such problems, improvement in power density of a fuel cell by increasing density in a power generation area, that is, a power generation area included in a unit volume of the fuel cell, has been aimed. For example, WO03/067693 (Patent Document 1) discloses a fuel cell layer, in which functions of a gas diffusion layer, a catalyst layer and an electrolyte layer are integrated into a single substrate, and proposes a fuel cell constituted of a smaller number of parts than in a conventional fuel cell or fuel cell stack having a layered planar structure.
More specifically, the fuel cell layer described in Patent Document 1 is connected to an external load with a fuel plenum, an oxidizing agent plenum, and a porous substrate communicating with the fuel plenum and the oxidizing agent plenum. The fuel call layer also has a porous substrate and numerous fuel cells formed using the porous substrate. Each fuel cell has a distinct channel, a first catalyst layer disposed on the first channel wall, a second catalyst layer disposed on the second channel wall, an anode formed from the first catalyst layer and an cathode formed from the second catalyst layer, and an electrolyte disposed in the distinct channel to prevent transfer of fuel to the cathode and to prevent transfer of oxidizing agent to the anode. The fuel cell also has a first coating disposed on at least a portion of the porous substrate to prevent fuel from entering a portion of the porous substrate, a second coating disposed on at least a portion of the porous substrate to prevent oxidizing agent from entering a portion of the porous substrate, a first sealant barrier disposed on the first side, a second sealant barrier disposed on the second side, a third sealant barrier disposed between the fuel cells, a positive electrical connection disposed on the first side, and a negative electrical connection disposed on the second side.
When a plurality of microscopically dimensioned fuel cells are formed within a single substrate, higher overall power densities can be achieved. In addition, as the multiple fuel cells within the single substrate can be formed in parallel, fuel cells of high capacity can be constructed. The combination of fuel cells within a single substrate minimizes the reliance on externally applied seals and clamps. A number of variations of the fuel cell layer are envisioned. Examples of the variations include having the fuel and oxidizing agent plenums dead-ended, having the fuel cell layer enclosing a volume, having the porous substrate in a non-planar, or alternately planar, configuration and having the fuel cell layer enclose a volume in a cylindrical shape.
The fuel cell layer described in Patent Document 1, however, requires separation between the oxidizing agent and the reducing agent for each of a surface and a rear surface of the fuel cell layer. In addition, in any fuel cell stack structure described in Patent Document 1, the fuel and the oxidizing agent should be supplied to the fuel cell layer in an in-plane direction. In a case where three or more fuel cell layers are stacked in a direction of layer thickness, the fuel plenum or the oxidizing agent plenum should be provided in order to supply the fuel and the oxidizing agent, which requires a prescribed interval (gap) between the fuel cell layers. Further, as the fuel plenum or the oxidizing agent plenum provided between the fuel cell layers should be sealed, a fastening member is required for ensuring sealing performance of a sealing material. Moreover, as supply and exhaust of the fuel and the oxidizing agent to the fuel cell layer in the in-plane direction is restricted, it is difficult to supply the fuel and the oxidizing agent through natural convection. In particular, as supply of air representing the oxidizing agent is difficult, auxiliary equipment requiring motive power such as a fan or a pump is required.
Namely, a fuel cell structure described in Patent Document 1 has suffered a problem of low power density when a stack structure in which fuel cell layers are stacked in a direction of layer thickness responds to demands for high output power on an equipment side. Meanwhile, when a stack structure in which fuel cell layers are stacked in an in-plane direction of the fuel cell layer responds to demands for high output power on an equipment side, arrangement of fuel cells on the equipment side and design of a mechanism thereof are much restricted, because the fuel cell layers require a large area in the in-plane direction.
Japanese Patent Laying-Open No. 5-41239 (Patent Document 2) discloses a high-temperature fuel cell module in which at least two stacks formed by stacking a plurality of cells constituted of an anode, a solid electrolyte, a cathode, and a ceramic separator form a part or the entirety of a manifold. According to such a configuration, effective use of an electrode area can be made and cost reduction can be achieved because the manifold can be formed with a simplified structure.
A fuel cell stack described in Patent Document 2, however, cannot three-dimensionally supply the fuel or the oxidizing agent into the fuel cell stack, as in the case of Patent Document 1 above. Therefore, such auxiliary equipment as a pump or a fan for supplying the fuel or the oxidizing agent at quite a flow rate is required, which results in a large-sized fuel cell system and increase in power consumption by the auxiliary equipment and hence low power density.    Patent Document 1: WO03/067693    Patent Document 2: Japanese Patent Laying-Open No. 5-41239